1. Field of the Invention
The present invention relates to an image signal processor for treating image signals from an image sensor as digital data.
2. Description of the Prior Art
In general, a composite video signal for color image is obtained by subjecting color component signals (R, G and B) representing three primary colors of red, green and blue to color difference matrix, balanced adjustment and so on. These processes are carried out by such an image signal processor as shown in FIG. 1, which is called a "color encoder".
A color separation circuit 1 is adapted to break down image signals (Y1), including color components of three primary colors (or combinations with their complementary colors) repeated in a given sequence, to the respective components in order to generate independent color component signals (R, G and B). The image signals (Y1) are provided by the outputs of an image sensor which may include a color filter comprising red-, green- and blue-color filters arranged in a mosaic and have a vertical scan cycle formed by a predetermined number of horizontal scan lines and a horizontal scan cycle formed by a predetermined number of image data. A white balance adjustment circuit 2 is adapted to provide a gain inherent in the respective one of the color component signals (R, G and B) to equalize the average level of each of the color component signals (R, G and B) such that the white color of a white-colored object can be reproduced on a reproduced scene. The white balance adjustment circuit 2 may be feedback controlled to approximate the integrated values of color difference signals (R-Y and B-Y), which will be described later, to predetermined values. A color difference matrix circuit 3 receives three types of color component signals (R, G and B) from the white balance adjustment circuit 2 and combines these color components in a given proportion (R:30%, G:59% and B:11%) to generate a brightness signal (Y:Y=0.3R+0.59G+0.11B). The circuit 3 is further adapted to subtract the brightness signal (Y) from two color component signals (R and B), in order to generate two types of color difference signals (R-Y and B-Y). A modulation circuit 4 is adapted to modulate the amplitudes of two color subcarriers (SC1 and SC2) different from each other by 90 degrees, with the color difference signals (R-Y and B-Y) so as to combine them to generate a chromatic signal (C).
A color burst signal generation circuit 5 is adapted to generate a color burst signal (CB). This color burst signal (CB) has a given phase difference from the color subcarriers (SC1 and SC2) with the same cycle as those of these color subcarriers and is generated through every 8 or 9 cycles at a given timing in the horizontal blanking term. An addition circuit 6 adds the chromatic signal (C) and brightness signal (Y) to the color burst signal (CB) and a composite synchronizing signal (CS) supplied from a synchronizing signal generation circuit 7, which will be described later, to generate an image signal (Y2) according to a television system. The synchronizing signal generation circuit 7 generates various types of sync-signals according to a reference clock (CK) which has a frequency defined by the television system such as NTSC, PAL or SECAM. These sync-signals are then supplied to the respective units to synchronize them in operation. At the same time, the synchronizing signal generation circuit 7 generates two different color subcarriers (SC1 and SC2) from the reference clock (CK), which are in turn supplied to the modulation circuit 4.
As shown in FIG. 2, the image signal (Y2) thus generated includes continuous image data corresponding to one horizontal line for every horizontal scan cycle and also includes a color burst signal (CB) and a horizontal synchronizing signal (HD) which are inserted into a horizontal blanking term partitioning between horizontal scan cycles adjacent to each other.
In recent TV camera systems or the like, it has been considered to replace the conventional image signal processors which use analog signal processing procedure with an image signal processor that uses a digital signal processing procedure which can be more easily adjusted and provides less degradation of the image signal. In such a case, the image signal (Y1) is subjected to analog/digital (A/D) conversion at the input step such that the color component signals (R, G and B) and color difference signals (R-Y and B-Y) can be handled as digital data during the respective signal processing procedures. After a given processing procedure has been completed, the signals are subjected to digital/analog (D/A) conversion to form the image signal (Y2).
It is to be noted herein that the image signal (Y2) shown in FIG. 2 is formed by image components (including brightness and color components) and synchronizing components (including scan timing and color synchronizations) which are different from each other in terms of the range of voltage to be taken. When the signal is handled as digital data, therefore, the gradation representing the image components becomes insufficient depending on the resolution of the D/A conversion circuit. Since the image and synchronous components of the image signal (Y2) are different from each other, the range of voltage substantially assigned to the image components becomes smaller than the range of reference voltage if the maximum and minimum voltages that can be taken by the image signal (Y2) are selected as reference voltages in the D/A conversion circuit. Even if the resolution of the D/A conversion circuit is sufficient, therefore, the gradation will be reduced by a reduced range of voltage assigned to the image components.